Not applicable
This invention relates to the measurement of the polarization of a xe2x80x9chyperpolarized resonantxe2x80x9d substance; that is, a substance whose nuclear or electron spins are not in thermal equilibrium (hyperpolarized), and which exhibit magnetic resonance.
This invention describes an instrument which can measure the polarization of any hyperpolarized resonant substance. However, the description of its operation will be illustrated using hyperpolarized 3He as an example.
There are two methods which have been used to measure the polarization of hyperpolarized 3He:
One is to apply a pulse of radio frequency energy at the resonant frequency of the 3He nuclei (e.g. M. Leduc, P. J. Nacher, D. S. Betts, J. M. Daniels, G. Tastevin and F. Laloxc3xa. Europhys. Lett. 4, 59-64 (1987)). This tips the polarization vector away from the direction of the guide field. The component of the polarization parallel to the guide field remains; the component perpendicular to the guide field induces a signal (the free induction decay) as it decays. The size of this signal is proportional to the polarization. If xcex8 is the angle through which the polarization vector is tipped, the sensitivity is proportional to sin xcex8, and the polarization remaining is proportional to cos xcex8. Typically useful values of xcex8 are 5xc2x0 to 10xc2x0. With xcex8=10xc2x0, half the polarization is lost after 47 measurements. This method can be performed using a standard MRI installation, but modifications are required which may be impossible on a commercially available machine. Apart from difficulties and inconveniences in handling the sample, this is very much like xe2x80x9cdoing the experiment and finding out afterwards what the polarization was.xe2x80x9d
Another method is to strike a discharge in the polarized 3He. This creates a mixture of ions and excited states which come into equilibrium with the ground state polarized 3He, and their polarization can be measured by optical means (J. M. Daniels and R. S. Timsit. Canad. J. Phys. 49, 539-544 (1971). M. Pinard and J. van der Linde. Canad. J. Phys. 52, 1615-1631 (1974)). This couples the hyperpolarized ground state to the outside world, and promotes rapid relaxation.
This method is not suitable for use at a remote site, since it essentially requires setting up the equipment to produce the hyperpolarized gas. In addition, the whole sample which is measured is usually destroyed.
The two methods, at present in use, for determining the polarization of a hyperpolarized substance suffer from the disadvantage that, in measuring the polarization, at least some of the polarization is destroyed. Often the polarization of the sample is totally destroyed, and the sample has to be discarded. For this reason, they cannot be used too often.
The invention described here is a self contained instrument for measuring the polarization of a sample of hyperpolarized material. It can be used independently of any other installation, and it causes minimal (essentially no) disturbance of the polarization of the sample. Continuous monitoring of the polarization is possible.
A use has recently been proposed for hyperpolarized noble gases in clinical Magnetic Resonance Imaging.
A hyperpolarized substance is unstable, and eventually relaxes back to thermal equilibrium. The time for hyperpolarized 3He to relax can vary between a fraction of a second and a week, depending on the conditions under which it is held. There is thus a need to be able to measure its polarization, and this will become more urgent when hyperpolarized 3He becomes a commercial commodity.
A situation is now envisioned where hyperpolarized noble gases are produced in a central location, and are shipped to distant sites for use, maybe on arrival, maybe at some future time within a few days. There are good reasons to be able to measure the polarization; for example, when the shipment arrives to verify that what has been sent is what was expected, or just before use to monitor the quality of the results.
This invention has, as its objective, a portable self contained instrument which can be used to measure the polarization with minimal (essentially no) destruction of polarization. This will permit continuous monitoring of the polarization, if desired.
The features of this invention which permit this are:
a. The specimen is in a radio frequency field, but this is a very weak field, not much stronger than the thermal circuit noise, in contrast to the xe2x80x9ctipping anglexe2x80x9d method which uses a strong pulse of radio frequency energy to tip the polarization vector, and
b. The frequency of this weak radio frequency field is always different from the resonant frequency of the spins of the hyperpolarized material being measured.
The Principle of Operation
The physical basis of this invention is the well known fact that two coupled resonant systems, (that is, mechanical or electrical systems which can perform free periodic motion) whose resonant frequencies are the same or almost the same, have two stable modes of oscillation. The frequencies of these two modes are different from each other, and also different from the resonant frequencies of the individual resonant systems. (Horace Lamb, xe2x80x9cHigher Mechanics,xe2x80x9d 2nd. Ed. Ch. 11, Sec. 90, pp. 219-222 Cambridge University Press (1943), Herbert Goldstein, xe2x80x9cClassical Mechanics,xe2x80x9d Ch. 16, Secs. 10.2 and 10.3, pp. 321-333 Addison Wesley (1950), Frederic Emmons Terman, xe2x80x9cRadio Engineers"" Handbook,xe2x80x9d pp. 154-164 McGraw Hill (1943)).
Another feature of such systems is that, when the system is incorporated in a feedback circuit so that sustained oscillations are set up, as the parameters of the system are varied, the system can jump from one mode of oscillation to the other. These jumps are seen frequently in radio circuits, due to non-linearities of the circuit parameters, and in circuits containing gas discharges. They have also been observed in a circuit containing a hyperpolarized gas. (R. S. Timsit and J. M. Daniels. Rev. Sci. Instrum. 47, 953-959 (1976)). It was recognized that this phenomenon was related to the polarization, however, this experiment has never been repeated, and the conditions and limitations of the use of this phenomenon for the measurement of polarization were not investigated.
Various embodiments have since been modelled on a computer (unpublished) and indeed this phenomenon can be used as a basis for a polarimeter.
There are several methods of detecting a mode switch, including, but not limited to, a change in frequency, and a change in the impedance of the resonant circuit resulting in a jump in the voltage across the circuit, or a jump in the current in the circuit. The polarization is most conveniently deduced from the difference in frequency of the two modes, or from the jump in frequency when the modes switch. This frequency jump can be displayed using conventional digital frequency counters, or an analogue frequency meter using the charging and discharging of a capacitor, or the output can be heterodyned into the audible range for an immediate and rough indication.
Most convenient would be automatic operation where the parameters of the oscillator are cycled through a predetermined sequence, and the frequency measurements are processed into a final result; this can be done with dedicated circuits, or an interfaced computer, or a combination of both.